Charge transfer band observed in bismuth mixed-valence oxides, Bi1-xYxO1.5+delta(x = 0.3)

The polycrystalline thin film of Bi1-xYxO1.5 (x = 0.3) has been prepared by r.f.-sputtering method and was annealed under various oxidation conditions for the introduction of excess oxygens. In Bi0.7Y0.3O1.5+delta, as the preparation condition became oxidative, the band gap decreased from 3.0 to 2.1 eV, indicating the formation of Bi5+. In pseudo-perovskite crystal, BaBiO3 thin film which was prepared for comparison, the intense and broad absorption band with the absorption coefficient of 105 cm(-1) located at 2.2 eV was observed. The absorption bands observed in both oxides were ascribed to the charge transfer transitions from Bi3+ 6s(2) to Bi5+ 6s(0). However, they had different intensities and energy positions. These transitions were classified into two types from the point of view of lattice vibration of anions between mixed-valence cations. In perovskite type in which (BiO6/2) octahedra are linked by sharing corner, the vibration of apical oxygens was expected to enhance the charge transfer transition. (C) 1997 Published by Elsevier Science Ltd.